The present invention relates to temperature sensors, and particularly to a sensor employing a thermistor which screws into an automotive engine block.
Automotive temperature sensors are generally in the form of a 3/8 inch taper threaded plug with the sensing end screwed into the engine block so it is in contact with the coolant in the case of a liquid cooled engine. One type of sensor employs a bulb filled with liquid and tubing connected to a gauge so that any temperature increase causes the liquid to expand which is translated into a continuously variable gauge reading. These are quite effective but expensive to manufacture and generally must be replaced as a system. Sensors employing bimetallic strips were developed where non-uniform expansion of the strip makes an electrical contact which activates an indicator light, but this sacrifices a continuous reading. The commonest type for a continuously reading gauge is that employing a thermistor. A thermistor is a solid state semi-conducting device whose electrical resistance varies with temperature. Common thermistors for automotive use have resistances on the order of 300 to 1000 ohms at 72 degrees F., the resistance decreasing as temperature increases to permit a current increase for a constant voltage drop, typically 12 volts. The thermistor is installed in the plug so that it is thermally and electrically grounded at the temperature sensing end and in contact with a conductor encased in the plug at the opposite or signal end, where a wire is attached and connected to a gauge where the amount of current passing through the wire translates into a continuously variable gauge reading. The plug is generally machined from brass and the conductor must therefore be insulated from the plug through the use of a sleeve and other fittings. A separate spring is utilized to maintain resilient contact with the thermistor. The prior art plug is provided with a bore in the signal end where the components are inserted and retaining means are placed while the sensing end is solid brass.